Neutron generator

Abstract

A neutron generator, comprising: (i) an ion source comprising an anode and a thermionic cathode disposed in an ionizable gas environment (e.g. hydrogen isotope); (ii) means for heating the cathode so that the latter emits electrons which, when colliding with the gas atoms, generate ions; (iii) a target; (iv) an electrical gap to accelerate ions from the ion source towards the target upon impingement of the ions; and (v) control means for applying voltages to the anode, cathode and electrical gap. The cathode is of the dispenser type or volume type, and preferably comprises one block of material comprised of a substrate impregnated with an electron emitting material.

What is claimed is: 
     
       1. A neutron generator comprising: (i) an ion source comprising an anode and a dispenser cathode disposed in an ionizable gas environment;   (ii) means for heating said cathode so that the latter emits electrons which, when colliding with said gas atoms, generate ions;   (iii) a target;   (iv) an electrical gap to accelerate ions from said ion source towards said target upon impingement of said ions; and   (v) control means for applying voltages to said anode, cathode and electrical gap, wherein a voltage applied to said anode by said control means is between 100 and 300 Volts to substantially reduce metal sputtering within the neutron generator.   
     
     
       2. The neutron generator according to claim 1, wherein said gas comprises at least one hydrogen isotope. 
     
     
       3. The neutron generator according to claim 2, wherein said gas environment constitutes a sealed chamber. 
     
     
       4. The neutron generator according to claim 1, wherein said cathode comprises at least one block of material comprised of a substrate impregnated with an electron emitting material. 
     
     
       5. The neutron generator according to claim 4 wherein said substrate is tungsten and said emitter material includes barium oxide. 
     
     
       6. The neutron generator according to claim 1, wherein said voltages are in the form of square voltage pulses. 
     
     
       7. The neutron generator according to claim 1 wherein said voltage applying means for said cathode is distinct from said cathode heating means. 
     
     
       8. The neutron generator according to claim 1, wherein said anode is made of a hollow elongated body permeable to electrons. 
     
     
       9. The neutron generator according to claim 8, wherein said anode is made of a cylindrical metallic coil. 
     
     
       10. The neutron generator according to claim 8, wherein said anode is made of a cylinder-shaped mesh. 
     
     
       11. The neutron generator according to claim 4, wherein said block is disposed at one end of an arm connected to said heating means and to said control means. 
     
     
       12. The neutron generator according to claim 8, wherein said cathode is disposed inside said anode. 
     
     
       13. The neutron generator according to claim 8, wherein said cathode is disposed outside said anode. 
     
     
       14. The neutron generator according to claim 11 wherein said cathode comprises two arms disposed diametrically on the outside of said anode. 
     
     
       15. The neutron generator according to claim 1, further comprising an extracting electrode disposed at the end of said ion source facing said target and submitted to a voltage complementary to the anode voltage. 
     
     
       16. The neutron generator according to claim 15, wherein the end of said extracting electrode facing said target is torus shaped. 
     
     
       17. The neutron generator according to claim 6, further comprising means for preventing slows ions, still present in said ion source at the end of said voltage pulse, from leaving said ion source. 
     
     
       18. The neutron generator according to claim 17 wherein said preventing means comprises a cut-off electrode disposed at the end of the ion source and which is submitted to voltage pulses synchronized with and complementary to pulses applied to said anode, and to a positive voltage between said pulses. 
     
     
       19. The neutron generator according to claim 18 wherein said cut-off electrode includes a mesh screen. 
     
     
       20. The neutron generator according to claim 19 wherein said mesh screen is in the form of a truncated sphere having its concavity facing said target. 
     
     
       21. The neutron generator according to claim 17 wherein said preventing means comprises means for applying to said extracting electrode negative voltage pulses synchronized with pulses applied to said anode, and a positive voltage between said pulses. 
     
     
       22. The neutron generator according to claim 3 comprising a cylindrical insulator disposed between said ion source and said target. 
     
     
       23. The neutron generator according to claim 22 wherein said insulator is made of ceramic. 
     
     
       24. The neutron generator according to claim 1 wherein said gas environment comprises a gas supply means incorporating a helical filament coated with material able, when heated, to emit atoms of at least one hydrogen isotope and disposed transversely to the longitudinal axis of the accelerating gap. 
     
     
       25. The neutron generator according to claim 23 wherein the gas pressure in said gas environment is comprised between 0.5 milliTorr and 20 milliTorr. 
     
     
       26. A neutron generator comprising: an ion source comprising an anode and a dispenser or volume type cathode disposed in an ionizable gas environment including at least one hydrogen isotope;   means for heating said cathode so that the latter emits electrons which, when colliding with said gas atoms, generate ions;   a target;   an electrical gap to accelerate ions from said ion source towards said target upon impingement of said ions; and   control means for applying voltages to said anode, cathode and electrical gap, wherein a voltage applied to said anode by said control means is between 100 and 300 Volts to substantially reduce metal sputtering within the neutron generator.   
     
     
       27. A logging tool for investigating earth formations surrounding a borehole, comprising a sonde incorporating at least one radiation detector and a neutron generator, said neutron generator comprising: (i) an ion source comprising an anode and a dispenser cathode disposed in an ionizable gas environment;   (ii) means for heating said cathode so that the latter emits electrons which, when colliding with said gas atoms, generate ions;   (iii) a target;   (iv) an electrical gap to accelerate ions from said ion source towards said target upon impingement of said ions; and   (v) control means for applying voltages to said anode, cathode and electrical gap, wherein a voltage applied to said anode by said control means is between 100 and 300 Volts to substantially reduce metal sputtering within the neutron generator.   
     
     
       28. A neutron generator for logging applications, comprising: a source of ionizable gas;   an ion source for ionizing said gas and including an anode and a dispenser type cathode designed to emit electrons able to impinge on gas atoms so as to generate ions;   a target spaced apart from said ion source by an accelerating gap, and being able to emit neutrons upon impingement of ions issued from said ion source;   control means for applying voltages to said anode, cathode and electrical gap; and   means for operating said control means such that the rise time for the neutron output to reach 90% of the maximum output plateau), measured from the time when the neutron output is 10% of said plateau, is less than 1 microsecond.   
     
     
       29. A neutron generator for logging applications, comprising: a source of ionizable gas;   an ion source for ionizing said gas and including an anode and a dispenser type cathode designed to emit electrons able to impinge on gas atoms so as to generate ions;   a target spaced apart from said ion source by an accelerating gap, and being able to emit neutrons upon impingement of ions issued from said ion source;   control means for applying voltages to said anode, cathode and electrical gap; and   means for operating said control means such that the time lag between the instant when the voltage is applied to said cathode and the instant time when the instantaneous neutron output reaches 10% of the maximum output (plateau), is less than 0.5 microsecond.   
     
     
       30. A neutron generator for spectral logging applications, comprising: a source of ionizable gas;   an ion source for ionizing said gas and including an anode and a dispenser type cathode designed to emit electrons able to impinge on gas atoms so as to generate ions;   a target spaced apart from said ion source by an accelerating gap, and being able to emit neutrons upon impingement of ions issued from said ion source;   control means for applying pulsing voltages to said anode, cathode and electrical gap; and   means for operating said control means such that the neutron output reaches a maximum value (or plateau) which remains constant within a 10% range thereof, over a pulse time width comprised between 18 and 25 microsecond.   
     
     
       31. A neutron generator for logging applications, comprising: a source of ionizable gas;   an ion source for ionizing said gas and including an anode and a dispenser type cathode designed to emit electrons able to impinge on gas atoms so as to generate ions;   a target spaced apart from said ion source by an accelerating gap, and being able to emit neutrons upon impingement of ions issued from said ion source;   control means for applying voltages to said anode, cathode and electrical gap; and   means for operating said control means such that the fall time between the instant when the voltage applied to said cathode is turned off and the instant time when the instantaneous neutron output falls to 10% of the maximum output (plateau), is less than 0.5 microsecond.   
     
     
       32. A neutron generator for spectral logging applications, comprising: a source of ionizable gas;   an ion source for ionizing said gas and including an anode and a dispenser type cathode designed to emit electrons able to impinge on gas atoms so as to generate ions;   a target spaced apart from said ion source by an accelerating gap, and being able to emit neutrons upon impingement of ions issued from said ion source;   control means for applying voltages to said anode, cathode and electrical gap; and   means for operating said control means such that the time required for the instantaneous neutron output to reach its maximum (plateau) value, measured from the instant time when the voltage is applied to said cathode, is less than 1.5 microsecond.   
     
     
       33. A method for investigating earth formation surrounding a borehole, comprising the steps of: irradiating, at a first given location in the borehole, the borehole materials and the earth formation with bursts of neutrons from a neutron generator including an ion source comprising an anode and a dispenser cathode disposed in an ionizable gas environment, by applying voltage pulses to the cathode and heating the dispenser cathode;   detecting, at a second given location in the borehole, radiation resulting from interaction of the neutrons with the formation;   generating signals representative of the radiation;   controlling the neutron output during the start of the neutron burst such that the rise time for the neutron output to reach 90% of its plateau, measured from the time when the neutron output is 10% of the plateau, is less than 1 microsecond; and   determining from the signals a characteristic of the earth formation surrounding the borehole.   
     
     
       34. A method for investigating earth formation surrounding a borehole, comprising the steps of: generating bursts of neutrons from a neutron generator including an ion source comprising an anode and a dispenser cathode disposed in an ionizable gas environment, by applying voltage pulses to the cathode and heating the cathode;   irradiating, at a first given location in the borehole, the borehole materials and the earth formation with bursts of neutrons;   detecting, at a second given location in the borehole, radiation resulting from interaction of the neutrons with the formation;   generating signals representative of the radiation;   controlling the neutron burst during the start of the neutron burst such that the time lag between the instant when the voltage is applied to the cathode and the instant time when the instantaneous neutron output reaches 10% of its plateau, is less than 0.5 microsecond; and   determining from the signals a characteristic of the earth formation surrounding the borehole.   
     
     
       35. A method for investigating earth formation surrounding a borehole, comprising the steps of: irradiating, at a first given location in the borehole, the borehole materials and the earth formation with bursts of neutrons from a neutron generator including an ion source comprising an anode and a dispenser cathode disposed in an ionizable gas environment, by applying voltage pulses to the cathode and heating the dispenser cathode;   detecting, at a second given location in the borehole, radiation resulting from interaction of the neutrons with the formation;   generating signals representative of the radiation;   controlling the neutron output such that the neutron output reaches a plateau which remains constant within a 10% range thereof, over a burst time width comprised between 18 and 25 microsecond; and   determining from the signals a characteristic of the earth formation surrounding the borehole.   
     
     
       36. A method for investigating earth formation surrounding a borehole, comprising the steps of: generating bursts of neutrons from a neutron generator including an ion source comprising an anode and a dispenser cathode disposed in an ionizable gas environment, by applying voltage pulses to the cathode and heating the cathode;   irradiating, at a first given location in the borehole, the borehole materials and the earth formation with bursts of neutrons;   detecting, at a second given location in the borehole, radiation resulting from interaction of the neutrons with the formation;   generating signals representative of the radiation;   controlling the neutron output such that the fall time between the instant when the voltage applied to the cathode is turned off and the instant time when the instantaneous neutron output falls to 10% of its plateau, is less than 0.5 microsecond; and   determining from the signals a characteristic of the earth formation surrounding the borehole.   
     
     
       37. A method for investigating earth formation surrounding a borehole, comprising the steps of: generating bursts of neutrons from a neutron generator including an ion source comprising an anode and a dispenser cathode disposed in an ionizable gas environment, by applying voltage pulses to the cathode and heating cathode;   irradiating, at a first given location in the borehole, the borehole materials and the earth formation with bursts of neutrons;   detecting, at a second given location in the borehole, radiation resulting from interaction of the neutrons with the formation;   generating signals representative of the radiation;   controlling the neutron output during the neutron burst such that the time required for the instantaneous neutron output to reach a plateau, measured from the instant time when the voltage is applied to the cathode, is less than 1.5 microsecond; and   determining from the signals a characteristic of the earth formation surrounding the borehole.   
     
     
       38. A method for investigating earth formation surrounding a borehole, comprising the steps of: generating bursts of neutrons from a neutron generator including an ion source comprising an anode and a dispenser cathode disposed in an ionizable gas environment, by applying voltage pulses to the cathode and heating the cathode;   irradiating, at a first given location in the borehole, the borehole materials and the earth formation with bursts of neutrons;   detecting, at a second given location in the borehole, radiation resulting from interaction of the neutrons with the formation;   generating signals representative of the radiation;   controlling the neutron output such that: (i) the rise time for the neutron output to reach 90% of its plateau, measured from the time when the neutron output is 10% of the plateau, is less than 1 microsecond; (ii) the fall time between the instant when the voltage applied to the cathode is turned off and the instant time when the instantaneous neutron output falls to 10% of the plateau, is less than 0.5 microsecond; and   determining from the signals a characteristic of the earth formation surrounding the borehole.   
     
     
       39. A method for investigating earth formation surrounding a borehole, comprising the steps of: generating bursts of neutrons from a neutron generator including an ion source comprising an anode and a dispenser cathode disposed in an ionizable gas environment, by applying voltage pulses to the cathode and heating the cathode;   irradiating, at a first given location in the borehole, the borehole materials and the earth formation with bursts of neutrons;   detecting, at a second given location in the borehole, radiation resulting from interaction of the neutrons with the formation;   generating signals representative of the radiation;   controlling the neutron output such that: (i) the rise time for the neutron output to reach 90% of its plateau, measured from the time when the neutron output is 10% of the plateau, is less than 1 microsecond; (ii) the fall time between the instant when the voltage applied to the cathode is turned off and the instant time when the instantaneous neutron output falls to 10% of its plateau, is less than 0.5 microsecond; and (iii) the neutron output reaches a plateau which remains constant within a 10% range thereof, over a pulse time width comprised between 18 and 25 microsecond; and   determining from the signals a characteristic of the earth formation surrounding the borehole.